Bleach activators are important ingredients in detergents, scouring salts and dishwashing agents. They permit a bleaching action even at relatively low temperatures in that they react with hydrogen peroxide--usually perborates or percarbonates--to release an organic peroxycarboxylic acid.
The bleaching result obtainable depends on the nature and reactivity of the peroxycarboxylic acid formed, on the structure of the bond that is to be perhydrolyzed and on the solubility of the bleach activator in water. Since the activator is usually a reactive ester or an amide, it is frequently necessary to use it in granulated form for the intended application in order to prevent hydrolysis in the presence of alkaline detergent ingredients and to ensure an adequate shelf life.
Numerous auxiliaries and processes have been proposed in the past for granulating these substances. EP-A-0 037 026 describes a process for producing readily soluble activator granules comprising 90 to 98% activator with 10 to 2% cellulose ethers, starch or starch ethers. Granules consisting of bleach activator, film-forming polymers and added organic C.sub.3 -C.sub.6 -carboxylic, hydroxycarboxylic or ether carboxylic acid are specified in WO 90/01535. EP-A-0 468 824 discloses granules comprising bleach activator and a film-forming polymer which is more soluble at a pH of 10 than at a pH of 7.DE-A-44 39 039 describes a process for producing activator granules by mixing a dry bleach activator with a dry, inorganic binder material containing water of hydration, compressing this mixture to form relatively large agglomerates, and comminuting these agglomerates to the desired grain size. A waterless production process, by compacting the bleach activator with at least one water-swellable auxiliary, without the use of water, is known from EP-A-0 075 818.
Disadvantages of these activator granules are that the properties of the granules are fixed essentially by the binder and by the granulating method used and that the resulting granules, besides the advantages described in the literature, often have certain disadvantages as well, for example suboptimal release of active substance, low abrasion resistance, high dust content, inadequate shelf life, separation within the powder or damage to the color of the fabric when used in detergents and cleaning materials.
In order to give granules defined properties a coating step is often carried out subsequent to the granulating step. Common methods are coating in mixers (mechanically induced fluidized bed) or coating in fluidized-bed apparatus (pneumatically induced fluidized bed).
For instance, WO 92/13798 describes, for a bleach activator, coating with a water-soluble organic acid which melts at above 30.degree. C., and WO 94/03305 describes coating with a water-soluble acidic polymer in order to reduce color damage to the laundry.
WO 94/26862 discloses the coating of granules consisting of bleach activator and a water- and/or alkali-soluble polymer with an organic compound melting at between 30 and 100.degree. C. for reducing separation in the pulverulent end product. In this case the activator granules are placed in a Lodige plowshare mixer, circulated at from 160 to 180 rpm at room temperature, without using the pelletizer, and then sprayed with the hot melt. A disadvantage of this process is the very poor coating quality, which, although it brings about a reduction in separation in the pulverulent end product, has no effect on the other granule properties, such as release of active substance, abrasion resistance, dust content or shelf life, for example. The positive effect on the separation behavior can probably be attributed to a droplet-like solidification of the coating substance on the granule surface allowing the individual grains to hook together in the bulk product.
The object of the present invention was to develop a coating process for activator granules which makes it possible to tailor the granule properties within a wide range at the same time as making optimum use of the coating material.
This object was achieved by a thermal conditioning during and/or after coating.